Amplifier of high-frequency electrical currents



June 26, 1928. 1,675,186 H. LOFTIN ET AL AMPLIFIER HIGH FREQUENCY ELECTRICAL GURRENTS Filed g m 7, 1923 Edward H. Lofii/n William F Grimes Patented June 26, 192 8.

I 1,675,186 UNITED STATES PATENT"OFFICE.

EDWARD H. LOFTIN AND WILLIAM F. GRIMES, OF WASHINGTON, DISTRICT OF COLUMBIA, ASSIGNORS TO ED'W .ARD H. LOFTIN, TRUSTEE, v

AMPLIFIER or HIGH-FREQUENCY ELECTRICAL cunnnnrs.

Application'filed June 7, 1923. Serial K0. 644,034.

Our invention relates generally to the amplification of high frequency electrical currents and is particularly adaptable to receiving radio signals. a

A particular object of our invention is the reception and efficient amplification of very feeble high frequency currents such as those encountered in long distance radio communications.

A specific object of our invention is the efficient use of thermionic vacuum tube amplifiers in tandem Without producing oscillations in the circuits of such amplifiers thereby preserving rather than destroying their functions. A further object is the combina tion of ordinary amplification obtained from a vacuum tube with that form of amplifica-' tion generally known as amplification by regeneration.

Great diificulty is had in endeavorin to use a number of vacuum tubes as ampli ers of high frequency electrical currents intandem to secure a high order of amplification on account of the reactive effects of the circuits of the various tubes on each other, which reactive effects tend to set up oscillations, particularly in the input circuit of the first tube. If such oscillations are set up they will beamplified .by the subsequent tubes, and if there are suflicient stages of amplification there will usually result a paralyzing of the action of the last tubes and no signals will be heard. This tendency to oscillate exists even Where there is no reactive or feed-back arrangement deliberately included somewhere in the system, and the control of this tendency naturally becomes much more diflicult if a feed-back arrangement is deliberately included in'the, system.

Our invention will be best understood by reference to the accompanying drawing in which the figure illustrates our invention employed in a s'ystem'in connection with. the reception of radio signals. It includes the combination 015a straight; radio. frequency amplifier reenforced by an amhaving regenerative action as an example of an arrangement for employing vacuum tubes in tandem for highly efficient selective high frequency current amplification, and including the provisions of our invention for maintaining efficiency While preventing oscillation production. Ais an" antenna grounded at G including in circuit therewith the tuning elements inductance L and Variable'capacity C The antenna is coupled through inductance L to the secondary circuit containing inductance L and variable capacity C VT is a three electrode vacuum tube including the secondary circuit in its input circuit and having a tunable circuit composed of in ductance L and variable capacity C, tightly coupled to its output circuit, as indicated by the full auto transformer or direct coupling across coil L The inherent giid-to-plate capacity, indicated, by capacity C in dotted lines, links the input circuit to the output circuit Battery B, provides for heating the filament of this vacuum tube, the amount of heating being controllable through variable resistance R The connection of the input circuit to the negative side of the filament provides for biasing the grid of this tube to cause it to operate effectively as a straight amplifier. Battery 13 supplies energy to the output circuit of the amplifier tube. r Vacuum tube VT has the tunable circuit containing inductance L and variable capacity 0 included in its input circuit and, as indicated by the illustrated tight coupling, is efliciently or output circuit of tube VT The output circuit includes inductance L variably coupled to inductance L for a regenerative action; that is, increase of amplification in tube VT and which is selective increase of amplification. by reason of the feed-back coupling to selective circuit L 0 Resistance R in parallel relation with capacity C, provides the commonly known grid leakstopping condenser combination. Battery provides for heating the filament of this second tube, the extent of heating being controllable through resistance 3,. The connection of the input circuit to the positive side of the filament provides for operating this tube asa detector, this in addition to its operation as an amplifier through regeneration. Battery. B provides a source of energy supply for the output. circuit-of this second tube. Telephones T provide for detecting-the s-i nals aurally. The condenser C provides a y-pass for the high trequency electrical currents around, the high impedance of the telephones.

We have found that by including in the output circuit of the first vacuum tube alinked to the amplifying choking .device D properly proportioned in its reactive effects for the, high frequency electrical currents contemplated for selective amplification in the illustrated system stable operation of the circuit arrangement herein described is-obtained, and that this is also possible even when employing a re generative action to sucha degree that high order of'amplification is secured. Without this device in the system illustrated when operated at the high frequencies and amplifications contemplated, or its equivalent in effect, it is in. practice impossible to use any amount of regeneration without setting the system into oscillations and destroying its function as a receiver. 1

An example of a simple and readily avail able reactive device we have found to give excellent results at the high frequencies ofv with the low natural period of the device.

. other apparatus broadcasting with which we have 'experi-f mented is a single ear pieces of an ordinary head telephone set having a direct current resistance of about 2,000 ohms. It includes a winding of many turns of very fine wire on an open iron core, and offers high impedance to currents of the high frequency order we have referred to; that is, sufiiciently high to be of the same order of magnitude as the internal output impedance of the type of tube we used in position VT -under the conditions.it was operated. The construction of this device inherently compels its impedance for the high frequenciesreferred to to be capacitive in nature, the effective high frequency path through the device being the istributed capacity path indicated by the capacity C in, dotted lines, this being because such telephones are used in connection frequencies of 'voice frequency currents, and though made to beresonant at a frequency above the practical range of voice frequencies to avoid distortion, yet this resonance is not" too far removed therefrom in order to preserve a practical degree of efficiency. This resonant point will in general be well below 100,000 cycles, and never reaches frequencies sufiiciently high for the small inherent tube capacity to become a, sufficient coupling for regenerative oscillation difficulties. In accordance with a well known principle all electrical devices react capacitively for frequencies above the The conductive path provided by the'coil admirably adapts the device for the use made of it, as

it permits of energizing the plate of tube VT from battery B and thus avoids use of and circuits for this purpose. j e

That the system functions to produce the results set forth under the conditions out lined and illustrated is readily appreciated by analyzing the actions and reactions that inherently exist therein with the aid of com- .from the antenna'current energy-o frequency and favorably vknovvn, ear piece of the pair of I monknowledge as tosome of the practical effects of them. Assume circuit L G set to absorb a given frequency. It will selectively the set offer it to tube VT,

for amplification with the natural amplifying ability of the tube as energized, with,

the result that amplified current of the selected frequency will predominate in the output circuit. Now adjust circuit L C to to selectively pass some of if eifort lpe made to this samefrequency the amplified energy on to tubeVT for further amplificatibn, the practical matter of varying the electrical constants in carrying out this-'eflfort will inevitably result in transiently pro ducingiin the output circuit of 'VT, the variety of reactions which circuit L G, is capable, and, as is well the reactions will be capacitive when the natural period of the circuit is be low the set frequency, inductive when above, and highly resistive'but non-reactive when in tune or accord, and at which setting the maximum ener y due to the natural am plifying ability of the tube would be delivered totube VT if inherent tube capacity C did not exist. The magnitude of these reactions will depend upon the degree of coupling, being the. stronger the tighter the coupling, and contemplated as tight in our system as indicated by the very tight coupling of the full auto transformer.

' Using the knowledge that. the grid-toplate-capaeity of commercial tubes sufiices as an effective coupling between the input and output circuits at high frequenciesto feed-back enough energy to greatly alter the natural amplification of the tube, and can artificially increase amplification ,to even produce oscillations, and that the fed back energy is of proper phase for this when the reaction of the output circuit is inductive to the set frequency, rather than non-reactive but resistive, as it is when circuit L G, is resonant and best adjusted to transfer the to the set frequency of set frequency, it is realized that circuit L,,C in the position shown, performs the two functions of selectively controlling amplification and selectively controlling energy transfer, maximum results from which are not had at the same adjustment. If the v coupling is so tight that oscillations are produced at the adjustment for maximum amplification then the system cannot be operated at this adjustment. As a ractical reaction on the output circuit sufficiently to i prevent oscillations the energy available for transfer may still be large but the efiiciency quently overall efficiency of transfer has been sacrificed.

'duced by its unmodified reaction, and propreventing the coupling reaction from viding means independent of couplin for being effective as an oscillation cause, but fully effective for energy transfer. .The impor-,

tance of this is readily apparent from practical considerations. Assume it is desired to have tube VT amplify the maximum amount below oscillation production and transfer the energy with a 50% coupling to a second tube, but it is found that without our system the coupling must be reduced to 25% before oscillations can be stopped. It is obvious that our system will transfer double the energywhich, when applied to the detector, takes advantage of the square law of detection to give an overall result four times as good" as the system that cannot use tight coupling WVith .a given coupling oscillation can be prevented by de creasing the natural amplifying ability of the tube through decreasing the energizing potentials, as by introducing energy consuming devices into ithe system. Such methods are undesirable for many reasons, and it will be noted that our system secures the desired result while avoiding both procedures.

Considering the inclusion in thesystem of regeneration in tube VT it is apparent that if the gnagnitude of the currentin circuit L 0 is increased by additional amplification in this manner the effective reaction of this circuit on the output circuit of tube VT, is

increased, so that if dependence were had on looseness of coupling to prevent oscillation it would be necessary to further loosen the coupling in proportion to the amount of regenerationused in orderto keep VT from oscillating. The fact that a high order of regeneration in VT can be used in company with tight coupling is practical proof of the effectiveness of our means for preventing oscillations.

The device being effective in keeping back oscillations in the combination such as we have illustrated, it is naturally suflicient for a combination not involving a deliberate regenerative action but having merely accidental or constructional reactions between the various stages of amplification or reactions of plate circuits through the internal capacities of the vacuum tubes.

Having described our invention, we claim:

1. In a system for efliciently amplifying high frequency electrical currents the combination of a three electrode vacuum tube amplifier having inherent capacity between its electrodes, means for energizing said tube to operate as an eiiicient amplifier, a high frequency tunable circuit connected to the inputelectrodes of said amplifier, a second'th-ree electrode vacuumftube having a turnable circuit in its input circuit-adapted to be varied in consonancewith said first tunable circuit, said second circuit being so closely coupled with' the output circuit of said firsttube to efliciently transfer energy therefrom that its transient reactions on said output circuit in the operation of varying it to follow said first tunable circuit-inevitably I,

include that reaction which unmodifiedly acting througlr said inherent tube capacity is of such phase and potential as to cooperate with said first tunable circuit to produce oscillation by regenerative action, and

means permitting maintaining said close coupling between said circuits and said eflicient amplification of said tube while preventing said oscillation including impedance means associated with a circuit of said first tube for modifying the effect through said inherent capacity of said coupling reaction to suflicient degree that said second circuit may be varied at will unaccompanied by oscillation production.

' 2. 'In a system for selectively and 'efiiciently amplifying high frequency electrical cur-- rents the combination of a three electrode vacuum tube amplifier, a high frequency tunable circuit connected to the input electrodes of said tube, a second three electrodevacuum tube having a tunable circuit in its input circuit adapted to be varied in consonance withsaid first tunable circuit, said second circuit being closely coupled with the output circuitofsaid firsttube to effi-' ciently transfer energy'therefrom, and an impedance element connected in said output circuit having impedance to high frequency currents for which the system is designed of the order of the internal output impedance of saidfirst tube. 4

3. In a system for efficiency amplifying high frequency .electrical currents the combination of a three electrode vacuum tube amplifier having inherent capacity between its electrodes, direct current means for energizing plate and filament electrodes .of said tube to operate as an efficient amplifier, a high frequency tunable circuit connected to the input electrodes of said three electrode vacuum tube having a tunable circuit in its input circuit adapted to be varied in c'onsonance with said first tunable circuit, said second circuit being so closely coupled with the output circuit of said first tube for efficiently transferring energy therefrom that its transient reactions on said output circuit in the operation of varying it to follow said first tunable circuit inevitably include that reaction which unmodifiedly acting through said inherent tube capacity is of such phase and potential as to cooperate.

tube, a second p I with said first tunable circuit to produce companied by oscillations by regenerative action, and means permitting maintaming said close coupling between said circuits and said efficient amplification of said tube while pre-' venting said oscillations including conductive impedance means associated with a cir cuit of saidfirst tube for modifyingitheeiiect through said inherent capacity fsaid coupling reaction to suflicient degree that said second circuit may be varied at will unacoscillation production and the tube electrode in said circuit may be energized by a direct current potential. f 4. In a system for efliclently amplifying high frequency electrical currents the. combination of a three electrode vacuum tube amplifier having inherent capacity between its electrodes, 'a tunable high frequency cir cuitconnected to the inputelectrodes of said tube, connections for operating said tube as an eflicient amplifier, a second three electrode I Vacuum tube having a, tunable circuit in its input circuit adapted to be varied in consonance with said first tunable circuit, said second circuit being so closely coupled with the output circuit of said first tube to efliciently transfer energy therefrom that its transient reactions on said output circuit in the operation of varying it to follow said first tunable circuit inevitably include that reaction which unmodifiedly acting through 4 said inherent tube capacity is of such phase oscillation production, ating said second tube frequency tunable circuit connected circuit in the and potential as to cooperate with said first tunable circuit to produce oscillation by regenerative action,.means for overcoming a desired degree of said regenerative action while permitting maintaining said eflicient amplification and said close coupling between said circuits whereby said' second circuit may be varied at will unaccompanied by and means for oper as a stable amplifier of the currentsdelivered thereto;

5. In a. system for efliciently amplifying high frequency electrical currents means for.

the multiple amplification of said currents including the combination of ,a' three elec-' trode vacuum tube amplifier having inherent capacity between its electrodes, a high to the Input electrodes of said tube, a second three electrode vacuum tube having a tunable 'circuit in its input circuit adapted to be varied in consonance with said firstitunable cir-" cuit said second' circuit being so -i.clos'ely coupled with the output circuit of said first tube to efliciently transfer energy therefrom that its transient reactions on said output operation of varying it to :follow said first tunable circuit inevitably include. that acting throu hfsaid inherent tube capacity is of such phase andpotential-a'sto' cooperateywith-said first tunable circuit to proselective stable amplifiers.

7. In an amplifier of high frequency elecjtunable circuit, said second circuit reactively associated with the output circuit reaction which immodifiedlyv duce oscillations by means for returning amp ified energy from the ,output circuit of said second tube to said second tunable circuit in degree sufficient to cause said second increase its amplifying ability without generating oscillations, but resulting in an in,- tensified reaction of said second tunable circuit on the output circuit of said first tube,

and means for preventing said first tube from generating oscillations while main-' I electrode vacuum tube amplifiers, a selective circuitconnected to the input electrodes-"of re enerative action,

tube to selectively one of said tubes, couplings linking the out-.

' put circuits of both of said tubes to said selective circuit whereby it performs the dual function of simultaneously selectively increasing the amplifying ability of both of said tubes, and means associated with both of said tubes for limiting the degree of said increase of amplifying ability whereby both of said tubes function as trical currents the combination of a three electrode'vac'uum amplifier having inherent capacity between its electrodes, a tunable circuit in the input circuit of said amplifier, a second three electrodevacuum tube having a tunable'circuit in its input circuit adapted v to be varied in consonance with said first being 'so of said first tube to efiicientlytransfer energy therefrom that its transient reactions on said output circuit in the operation of varying it to follow said first tunable circuit'inevitably include that reaction of inductive nature which unmodifiedly' acting through said inherent tube capacity produces .oscil- I lation by regenerative action, and a capacitively reacting element associated with said output circuit to alter theoverall reaction thereof to suflicient degree to'prevent said oscillation production while maintaining said coupling relation between said circuits, may be varied at Wlll unaccompanied by oscillation production.

; 8.iIn an'amplifier of high frequency elec-- trical currents the combination of a three electrode vacuum tube amplifier having inwherebythe tuning of said second circuit herent capacity between its electrodes, a

tunable circuit in the input circ'uitof said amplifier, a second three electrode vacuum,

tube having a tunable circuit in its input \ciently transfer energy I v29 sary to tunable circuit in the input circuit adapted to be varied in consonance with said first tunable 'circuit, said second circuit being so reactively associated with the output circuit of said first tube to eflitherefrom that its ransient reactions on said output circuit i evitably include that reaction of inductive nature which unmodifiedly acting through said inherent tube capacity produces oscillation by regenerative action, and means for preventing said oscillation production while maintaining said eflicient coupling comprising a coil in said output circuit so wound that its reaction to all the current frequencies adapted to be selected'by said tunable circuits is inevitably capacitive, and is sufficient to oppose said inductive reaction to adequate degree to modify the overall out put circuitreaction by that amount necesprevent oscillation production, whereby the tuning of said second circuit may be varied at will unaccompanied by oscillations.

9. In an amplifierof high frequency electrical currents the combination of a three electrode vacuum tube amplifier having in-: herent capacity between its electrodes, a circuit of said ciently transfer energy amplifier, a second three electrode vacuum tube having a tunable circuit in its input preventing said oscillation production while ing a reactance element so connected in said output circuit as not: to prevent the .flo'w of direct current therein but presenting a reaction to all the current frequencies adapted output circuit maintaining said eflicient coupling compris I to beselected by said tunablecircuits that is sufficiently capacitive to oppose said in ductive reaction to adequate degree to mod ify the overall output circuit reaction by that amount necessary to prevent oscillation production, .whereby the tuning of said second circuit may be varied at will unaccompanied by oscillations. v i

' EDWARD H. LOFTIN. it WILLIAM F. GRIMES. 

